A few notes about

TRANSFORMERS

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Every model railroad needs some sort of power supply. The first power supply for a model railroad was contained in a box referred to as a transformer. To be technical it was a bit more than that but in fact it was more that than anything else by weight and that was its main function. The box also contained a speed control, a way of reversing the train, some sort of circuit breaker, and a few things more. The transformer remains the key element of a power supply as shown below. The other components, if included in the circuit, remain inside the same box and included in other parts of the circuit. Only the parts that comprise the power supply are shown here.
The Transformer, as far as model railroading is concerned, is an AC device used to change dangerous 112 volts house current to a much safer 18 volts at a much lower current level. voltage levels and/or isolate circuits. current is transferred from one wire (primary) to other (secondary) a magnetic core is used to increase the efficiency of this transfer extra connections added for flexability center tap primary primary wire has extra connection twin coil primary for 220 vs. 110 AC input secondary tapped or split like primary sometimes series for voltage sometimes parallel for current sometimes unequal or multiple for various voltages for other circuits secondaries electrically isolated inside beware the autotransformer it is possible to have a single priamary tapped to provide secondary it works but dangerous – no isolation from outside current benefits of transformer isolates dangerous power levels change voltage level step up step down
types of transformses Plug-in wall transformers – hand for small amounts of current Chasis mount used where more power is desired PC mount – in between plug-in and chasis mount Toroidal a specialized donut-shaped ring More expesive, more efficient,
Selecting the correct transformer for a project can be tricky First, look at specs Voltage Amps, Volt amps A 18v transformer measures 24v AC DC volts * 1.4 = AC volts Transformer drops as load applied Transformers get warm at rated voltage and need ventilation Outside factors Each stage of power supply AC house current can vary Fuse has tiny voltage drop Transformer output voltage drops under load Diodes of rectifier add their amount Typical drop is .4v Voltage regulators require (3) volt drop Over this contributes to overheating Less may cause erratic performance Further drop in throttle circuits Junctions each add their drop in final stage Contacts of reversing relay etc all add their drop Layout wiring resistance Best bet Start with external power supply Build circuit Measure required values Then build power supply Test and verify as required
Transformers

Fuses

No fuse will protect any solid state component from damage

Proper size transformer will minimize risk

But will provide margin of safety

Should not be ignored

Bi-metalic stip

Different metals expand at different ratewsw

Current warms spring

Spring trips at rated value

Grasshopper fuse

Simplest and cheapest

Can be adjusted Grasshopper fuse

Pre-set

More precise and admustable

Limited number of trips

Metallic strip fuse

Most common

One use only

Sealed breaker

Trips at rated current

Automatic resets

Recitfier, capacitor, and resister

A diode allows current to flow in on direction only

Can be used AC to Convert to DC

Two can improve flow

Four diodes make up rectifier

Converts AC to DC more efficiently

C1 is required for electronics

Prevents DC from dipping to zero

Holds closer to AC levels

R1 and LED1 serve two purposes

Warn user that circuit is active and in use

Imply AC may be present

Quickly drain capacitor

Prevent nasty shocks when working with circuit

Regulator The is a black box Takes many forms Single component to built up circuits
Switching power supplies Mainstay of computers Smaller, cooler, much more efficient, more complex This dual power supply illustrates use of toroidal transformer Details Not in scope of this article

This Power cage (formerly a steel 1950’s hi-fi amplifier) supplies the entire electrical needs for the North River Railway. Note the fuse in the upper right corner and the switching voltage regulators in the second bay of the cage. The three transformers in the bottom bay proved throttle power.

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Bob Van Cleef, MMR

Last update
10/25/2008